You should definitely go to Visitas and Bulldog days to try to get more of a feel for which campus/environment you would prefer. However, I would agree with gibby about flagship state schools if you’re really interested in medicine (although I realize almost no kid will take this advice no matter how wise it is).
In terms of whether Harvard or Yale is better for premed, it’s a toss-up. It boils down to wherever you are going to be more comfortable at and therefore probably do better academically. If you are looking for a nurturing environment, then Harvard is probably not for you. That being said, my daughter is thriving at Harvard and could not imagine herself at other school.